The present invention relates to a method and apparatus for introducing a material into a sample of molten metal. More specifically, the present invention relates to a method and apparatus for introducing a carbide stabilizer, such as tellurium and/or bismuth into a metallurgical sample of cast iron or blast furnace hot metal and for homogeneously distributing the stabilizer throughout the sample to induce white solidification.
During the processing of metal making, and in particular, during the molten processing of cast iron or blast furnace hot metal, it is often desirable to obtain samples of the metals for subsequent metallurgical analysis for ascertaining the precise chemical composition of the metal. With respect to certain grades of cast iron containing high concentrations of carbon and/or silicon and phosphorus, and especially blast furnace produced hot metal, the resulting solidified structure of a metallurgical sample will generally be in a form known in the industry as "gray iron". Gray iron refers to iron having enhanced graphite formation. Under certain circumstances, it is desirable for the metallurgical sample to have a "chilled" structure, also known in the industry as "white" iron. White iron refers to iron having enhanced carbide formation, and the process of forming white iron is called white solidification. A multitude of devices for collecting samples from a bath of molten metal have been proposed heretofore. A typical device comprises a molten metal sample cavity or chamber constructed from a suitable refractory material, such as foundry sand and cement. U.S. Pat. No. 3,452,602 discloses a typical collection device comprising a generally tubular conduit extending from the sampling cup or chamber, and is incorporated herein by reference. Upon insertion of the probe into a bath of molten metal, the molten metal is conveyed to the sampling cup or chamber via the conduit.
It is generally well known in the industry that a molten metal sample which would normally solidify as gray iron may be caused to solidify as white iron by using heavy cooling plates or particular sample cavity shapes of molten metal in the construction of the molten metal sampling chamber which promote white solidification. By using such cooling plates or cavity shapes, there is caused a generally rapid rate of solidification of the molten metal sample, promoting a white solidification structure. It is this rapid cooling process which gives rise to the term "chilled iron" when referring to white iron.
The problem associated with this type of approach is that the sample obtained may not be uniformly white throughout. I.e., within the same sample, there may be obtained a white or chilled surface layer, a mottled transition zone having both graphite and carbide structures and a gray iron interior. The depth of the white chilled surface layer may also vary, depending upon the particular metal composition and cast temperature, the structure and/or heat capacity of the chilling plates or the cavity structure. For example, graphite formation promoters within the molten metal and variations in the temperature of the molten metal at the time of sampling may cause variations in the amount of white chilled iron obtained. Each individual metallurgical sample must be separately prepared for analysis after solidification, for example, by the surface grinding of at least a portion of the sample for spectrometric or x-ray fluorescence analysis If the depth of the white chill layer in the sample is insufficient to provide a white chilled structure after the grinding and polishing steps, significant errors in the sample analysis may occur.
It is generally known in the art that various alloying elements may be used as stabilizing additives or stabilizers to enhance the tendency of gray iron to solidify as white iron. Such alloying elements have been used in sampling devices and sensors, such as phase detecting thermal analysis solidification cups, to promote white solidified iron. In addition, U.S. Pat. No. 3,546,921 discloses a method of producing the white chilled structure by introducing such a stabilizing additive for retarding primary graphite formation as the molten metal sample cools. This stabilizing additive is generally selected from one or more elements of bismuth, boron, cerium, lead, magnesium and tellurium.
There are a number of inherent problems associated with the existing methods employed for adding any of the aforementioned stabilizing additives to a molten metal sample for promoting white solidification. For example, it is generally necessary to insure that an adequate amount of the stabilizer is present in the molten metal sample to promote the formation of the white iron over a full range of sampling temperatures and chemistries, as mentioned in U.S. Pat. No. 4,059,996. If the temperature of the molten metal is too high, some or all of the stabilizer may burn or vaporize, resulting in a low efficiency of the additive and hence, only partial white solidification. If the molten metal sampling temperature is too low, the sample may solidify before becoming thoroughly mixed with the stabilizer, again resulting in only partial white solidification.
In addition, the stabilizer must be added in a manner providing generally uniform distribution of the stabilizer through the body of the sampling device. This uniform distribution is necessary to obtain a total white solidified cross section. Similar problems in the addition of deoxidants in molten steel sampling have been addressed in U.S. Pat. No. 4,037,478.
The present invention addresses the prior art problems of obtaining generally fully chill promoted white solidified molten metal samples and in homogeneously distributing a stabilizing additive throughout a molten metal sample by providing a mechanism for releasing a controlled amount of stabilizer into a sample of molten metal as it flows into the sample cavity without major modifications to any existing sampling device.
The present invention achieves the desired result by sandwiching the stabilizing additive between two layers of a melting material which melts at a known rate or by alloying or admixing the stabilizing additive with a melting material which melts at a known rate to provide a single layer device. The melting material may be a material having a high melting temperature, a low melting temperature or a temperature therebetween. Preferably, the melting material is a high melting temperature material, such as low carbon steel. Upon immersion of the sandwich device into a molten metal bath, the two layers admix or alloy with the molten metal at a controlled rate to expose the stabilizing additive to the molten metal flowing into the sample cavity. Upon immersion of the single layer device into a molten metal bath, the layer admixes or alloys with the molten metal at a controlled rate to alloy the stabilizing additive with the molten metal flowing into the sample cavity. The sandwich device and the single layer device are comprised of a cap member which is small enough to be located at the inlet or sampling chamber apparatus in the same manner as a protective inlet cap member which is currently used in prior art sampling devices.